1. Field of the Invention
This invention relates to mixing of audio signals and more specifically to a mix or downmix of two or more audio channels using a correlated output.
2. Description of the Related Art
Multi-channel audio has received enthusiastic acceptance by movie watchers in both traditional theater and home theater venues as it provides a true “surround sound” experience far superior to mixed stereo content. Dolby AC3 (Dolby digital) audio coding system is a world-wide standard for encoding stereo and 5.1 channel audio sound tracks. DTS Coherent Acoustics is another frequently used multi-channel audio coding system. DTS Coherent Acoustics is now being used to provide multi-channel music for special events and home listening via broadcast, CDs and DVDs 5.1, 6.1, 7.1, 10.2 and other multi-channel formats
Car audio systems have over the years advanced from mono to stereo to the multi-speaker systems standard in most every automobile today. However, most content is still provided in a 2-channel stereo (L,R) format. The audio system mixes and delays the two channels to the multi-speaker lay out to provide an enhanced audio experience. However with the growing availability of multi-channel music, multi-channel audio systems are being implemented in automobiles to provide passengers with a “surround sound” experience.
Although a significant improvement over existing audio systems, the confines of the car and proximity of passengers to particular speakers affect the surround-sound experience. In general, the desired mix embodied in the multi-channel format may become “unbalanced”. For example, a passenger sitting in the front passenger's seat may here too much of the discrete R channel that is emanating from the front right speaker effectively losing some of the benefits of the surround sound presentation. Even more extreme, a passenger in the back seat may here only the surround sound channels.
As a result, automakers have found that some amount of remixing of the discrete channels can reestablish the desired balance and improve the surround sound experience for everyone in the car. As shown in FIG. 1, a typical mixer 10 remixes the discrete R,C,L input channels 12,14,16 into R,C,L output channels 18,20,22 for an automobile. Each channel is passed through a delay 24 and mixed (multiplied by gain coefficients Gi 26 and summed 28) with the adjacent channels. Standard mixing equations are:R=G1*R+G2*CC=G3*C+G4*L+G5*R, andL=G6*L+G7*C.The mixed channels are passed through equalizers 30 to the output channels 18,20,22 for playback on the L,C,R channel speakers in the automobile.
Although this approach is generally effective at rebalancing the audio to provide a reasonable surround-sound experience for every passenger in the automobile there are a few potential problems. This approach may introduce unwanted artifacts when two channels include the same or very similar content but with a relative time or phase delay. Furthermore, this approach may over mix the signals that were assigned to a specific channel thereby degrading the “discreteness” of the multi-channel audio.